The present invention relates generally to Precision Approach Path Indicator (PAPI) visual guidance systems for aiding pilots in landing an aircraft. Specifically, the PAPI system defines a desired vertical approach angle to a runway and indicates to the pilot, via colored lights, whether the angle of approach of his aircraft to the runway is within the desired approach angle. The colored lights are produced in a number of Lamp Housing Assemblies (LHAs), as will be described below. The Federal Aviation Administration (FAA) establishes the standards for PAPI systems in the United States, whereas the standards for foreign PAPI systems may differ. Therefore it should be understood that, while the present invention is described with respect to the FAA endorsed systems, its application should not be considered as limited to FAA endorsed systems.
The components in an FAA Style B PAPI system are powered by the well-known and widely used constant alternating current (AC) loop employed in most of the world's airport lighting systems, whereas the components in an FAA Style A PAPI system are powered in parallel, directly from utility line power. In any PAPI system there are a number of important considerations, among them being: power consumption; number and type of lamps; size of the Lamp Housing Assemblies (LHAs); system reliability; ease of installation and service; safety with respect to exposed wiring and high voltages; ease of detection and identification of lamp/housing problems; environmental impact of components used; and minimization of the number of wires and interconnections.
The PAPI system generally comprises an array of two or more LHAs, each of which may contain two or three individual lamps. The LHAs are located adjacent the side of a runway and precisely aimed to define a correct vertical approach angle for guiding an incoming aircraft. Generally, each LHA is fitted with an optical filter to present a white light when the aircraft is too high, i.e., above the correct approach angle, and a red light when the aircraft is too low or below the correct approach angle. When the aircraft is too high, all of the LHAs are seen as white lights, when the aircraft is too low, all of the LHAs are seen as red lights and when the aircraft is within the correct approach angle, one-half of the LHAs in the array present a white light and one-half present a red light. The PAPI system usually comprises either two LHAs or four LHAs, with each LHA having either two lamps or three lamps. A two LHA system will therefore show: two red lights when the aircraft is too low; one white and one red light for a correct approach; and two white lights when the aircraft is too high. A four LHA system will also indicate intermediate positions within the correct approach angle. Thus the light indications will be: four red for too low; one white and three red for slightly low; two white and two red for correct approach angle; three white and one red for slightly high; and four white for too high.
The PAPI system also includes a tilt detection arrangement and tilt switch control circuitry for disabling the entire LHA array should the physical attitude or position of any of the LHAs be displaced by a predetermined amount for a predetermined time. This is necessary since the color of the light seen by the pilot could be erroneous and create a potentially hazardous situation should an LHA position be disturbed sufficiently to change its aiming. The choice of PAPI system selected is determined by a number of factors, such as airport size and location, traffic density, economics and the like. For example, some airport installations use multiple PAPI systems located at differing distances (touch down points) along the runway to accommodate aircraft having different landing requirements. The present invention is useful in all PAPI Style A systems.
Current state-of-the-art FAA Style A PAPIs supplied by Multi Electric Mfg. Inc., the assignee of the present invention, include two or more LHAs, each of which is mounted above ground adjacent to the runway and connected to a master control via underground conduit or buried cabling. It will be appreciated that a minimum number of wires and connection points in the PAPI system is a desirable objective with respect to cost, installation and maintenance. Also, it is desirable to minimize the amount of above-ground equipment to avoid damage to or from vehicles and aircraft. The LHAs are connected through break-away type connectors that are designed to readily separate in the event of contact with a moving object or vehicle. The connectors are also poled to minimize exposure of high voltages in the event of a connector separation.
The system of the invention uses optical pendulum tilt switches that require a power connection for a light-emitting diode (LED) and a signal connection for a photo transistor detector which, but for the invention, would undesirably add to the number of wires and connections. The arrangement provides a visual indication of a tilt condition, identifying the particular LHA or LHAs involved, at both the master control and the LHA, reduces system cost and complexity and enhances system manufacture, installation and service.